There is a growing need for a range of positive-acting imaging materials able to complement those provided by negative-acting Dry Silver systems, particularly for use in graphic arts and medical imaging. For such uses, prospective imaging materials should desirably exhibit the following characteristics:
(i) contact (10.sup.5 to 10.sup.3 ergs/cm.sup.2) or higher speeds; PA1 (ii) visible through to near infrared (VIS-NIR) spectral sensitisation for higher speed materials; PA1 (iii) imaged areas should be transparent to ultraviolet through to visible light (UV-VIS) (340 to 700 nm) so that the exposed film can be used for subsequent contact applications in a printing process; PA1 (iv) dry processing, preferably using standard Dry Silver processing conditions, and PA1 (v) be image stable, for subsequent contact exposure.
The prior art has long sought to provide suitable positive-acting photothermographic materials, examples of which are disclosed by: British Patent Nos. 1156933, 1172425, 1507829 and 2022277; European Patent No. 320020; U.S. Pat. Nos. 3,589,901, 4,075,017, 4,124,387 and 4,587,198, and Japanese Patent Nos. 53-120520, 57-089750, 57-101832, 58-040543, 58-040544, 61-107243, 62-187837, 62-187842, 63-034536 and 63-330064.
European Patent Publication No. 362827 discloses a positive-acting photothermographic element having a photosensitive medium comprising a heat-transferable colourant, a polymerisable monomer and a photoinitiator therefore. Upon exposure of the element to actinic radiation, polymerisation occurs in the light-struck regions of the element, thereby immobilising the colourant in those regions and generating a latent image. The exposed element is then contacted with an image receptor sheet and heated to transfer in an imagewise fashion colourant from the element to the receptor in the non-exposed regions of the element.
Japanese Patent No. 60-030931 discloses a positive-acting photothermographic element comprising as a photosensitive medium therefor, layers of heat sensitive colour forming reagents and a photopolymerisable monomer. Following exposure to light, photopolymerisation occurs in the light exposed regions, thereby immobilising one of the colour forming reagents and preventing it from mixing with the remaining components of the colour forming system during thermal processing. There is no discussion of the importance of choice of photoinitiator for generating positive, neutral images or of the preparation of compositions which do not absorb near UV light (340 to 400 nm) in the imaged areas of the element.
European Patent No. 223587 and U.S. Pat. Nos. 4,772,541, 4,800,149 and 4,865,942, disclose positive-acting photothermographic elements comprising microcapsules, each containing a monomer, a photoinitiator and a dye precursor or developer, dispersed in a binder layer. When the microcapsule containing layer is exposed to light of the appropriate wavelength, the monomer polymerises so as to immobilise the dye precursor or developer in the exposed regions. The exposed element is processed by contacting the element with an image receptor sheet under pressure and heating to develop the final image. The photoinitiator consists of an ionic dye-counter ion complex, such as a cationic dye-borate anion complex or an anionic dye-iodonium ion complex. There is no discussion of the necessity for producing images which are transparent to near UV light in the exposed regions. The only photobleachable sensitisers disclosed are 3-substituted coumarin derivatives which have a sensitivity within the range of 390 to 500 nm.
U.S. Pat. No. 4,761,360 discloses photothermographic elements having as a photosensitive medium therefor, a microencapsulated formulation comprising silver halide, a reducing agent, a free radical polymerisable compound, silver benzotriazole and a silver diazotate compound. The latent image centres produced in the exposed regions of the element act as a catalyst for the reduction of the diazotate compound to a diazohydroxide during development. The diazohydroxide decomposes on heating to produce free radicals which initiate polymerisation in the imaged areas. U.S. Pat. No. 4,753,862 discloses photothermographic elements of similar construction but which incorporate silver triazene compounds instead of silver diazotate.
Japanese Patent Nos. 61-183460, 61-188535 and 61-022841, European Patent Publication No. 301539, British Patent Publication No. 2195463 and U.S. Pat. No. 4,814,252 also disclosed positive-acting photothermographic elements which utilise polymerisation in light struck areas to generate an image. However, the preferred method of image formation is achieved by heating the photosensitive layer in contact with an image receiving layer. The photothermographic elements are usually colour-forming.
The present invention seeks to provide alternative positive-acting photothermographic materials which are suitable for use at contact speeds.